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Applied and Environmental Microbiology, February 2003, p. 901-908, Vol. 69, No. 2
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.2.901-908.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Evaluation of Broiler Litter with Reference to the Microbial Composition as Assessed by Using 16S rRNA and Functional Gene Markers
Jingrang Lu,1 Susan Sanchez,1,2 Charles Hofacre,3 John J. Maurer,3 Barry G. Harmon,4 and Margie D. Lee1*
Medical Microbiology and Parasitology,1
Athens Diagnostic Laboratory,2
Avian Medicine,3
Veterinary Pathology, College of Veterinary Medicine, The University of Georgia, Athens, Georgia 306024
Received 8 July 2002/
Accepted 25 November 2002

ABSTRACT
Very little is known about the microbial composition of animal
bedding wastes, including poultry litter, and what is known
has been deduced from standard culture methods, by which some
fastidious organisms that exist in the environment may not be
detected. We evaluated the bacterial composition of poultry
litter by using a combination of culture and molecular detection.
Total aerobic bacteria in poultry litter were detected by culture
at 10
9 CFU/g of material. Enteric bacteria such as
Enterococcus spp. and coliforms composed 0.1 and 0.01%, respectively, of
the total aerobic cultivatable bacteria in poultry litter; no
Salmonella strains were detected by culture. In order to characterize
the most abundant bacterial groups, we sequenced 16S ribosomal
DNA (rDNA) genes amplified by PCR with microbial community DNA
isolated from poultry litter as the template. From the 16S rDNA
library, 31 genera were identified. Twelve families or groups
were identified with lactobacilli and
Salinococcus spp. forming
the most abundant groups. In fact, 82% of the total sequences
were identified as gram-positive bacteria with 62% of total
belonging to low G+C gram-positive groups. In addition to detection
of 16S rDNA sequences associated with the expected fecal bacteria
present in manure, we detected many bacterial sequences for
organisms, such as
Globicatella sulfidofaciens,
Corynebacterium ammoniagenes,
Corynebacterium urealyticum,
Clostridium aminovalericum,
Arthrobacter sp., and
Denitrobacter permanens, that may be involved
in the degradation of wood and cycling of nitrogen and sulfur.
Several sequences were identified in the library for bacteria
associated with disease in humans and poultry such as clostridia,
staphylococci, and
Bordetella spp. However, specific PCR targeting
other human and veterinary pathogens did not detect the presence
of
Salmonella, pathogenic
Escherichia coli,
Campylobacter spp.,
Yersinia spp.,
Listeria spp., or toxigenic staphylococci. PCR
and DNA hybridization revealed the presence of class 1 integrons
with gene cassettes that specify resistance to aminoglycosides
and chloramphenicol. Only from understanding the microbial community
of animal wastes such as poultry litter can we manage animal
disease and limit the impact of animal waste on the environment
and human and animal health.

INTRODUCTION
Poultry litter is a mixture of excreted manure mixed with bedding
material. According to the U.S. Food and Drug Administration,
5.6 million tons of litter dry matter is produced per year in
the United States (
16). One type of poultry waste is broiler
litter resulting from the production of commercial broiler chickens
on wood chips, sawdust, wheat straw, peanut, and rice hulls
(
61). Analysis has shown that broiler litter contains potentially
valuable plant nutrients, including nearly 30% crude protein
and high levels of minerals and some heavy metals (
36). For
this reason, litter is commonly used as a fertilizer but environmental
concerns, such as nutrient, nitrate, and phosphate runoff to
streams, ponds, and groundwater have limited its acceptability
(
53,
54). Another disadvantage is the tendency for ammonia and
other volatile gases to escape into the atmosphere, resulting
in unpleasant odors.
In addition to its use as fertilizer, poultry litter has nutritional value in cattle. Growth performance for cattle on feed supplemented with poultry litter is similar to or higher than a diet with good-quality legume hays (53). The U.S. Food and Drug Administration estimates that 20 to 25% of the broiler litter is fed, but there are concerns regarding the safety of feeding large quantities of poultry litter to cattle, which are susceptible to infection by pathogenic microorganisms such as Listeria monocytogenes and Salmonella and Campylobacter spp. that may be present (36). Botulism outbreaks have also been described in cattle herds fed contaminated litter (44).
These concerns necessitate knowledge about the composition of the bacterial microflora present in broiler litter. Martin et al. performed a survey of pathogenic microorganisms in poultry litter with selective medium and found Staphylococcus xylosus to be a predominant species (36). However, we have not yet cultured the majority of the bacteria in the environment because many microorganisms are difficult to culture (60, 63) and selective media are often not truly specific or are too selective for certain bacteria (23, 42, 65). In the present study, we characterized the bacterial composition of broiler litter in order to assess its safety as fertilizer or feed amendment for cattle, employing 16S ribosomal DNA (rDNA) sequencing and PCR screens for pathogens and antibiotic resistance genes.

MATERIALS AND METHODS
Origin and collection of litter samples.
Litter was collected from four geographically separate farms
in Northeast Georgia and poultry houses already stocked with
broiler chickens. Birds present on these four farms ranged in
age from a few days old to 6-week-old, market-weight size animals,
and the litter had not been removed from the house between flocks.
All farms were contracted to raise birds for a single poultry
company. The company provided the farm with the birds and feed.
Wood shavings from softwoods were used as bedding; material
commonly used in poultry houses in the southeastern United States.
Litter samples were randomly removed from underneath the nipple-drinker,
water lines, and along the length of the flock house and then
pooled. Five such samples were collected from each house. Five
grams of chicken litter was resuspended in 30 ml of phosphate-buffered
saline and then mixed for 5 min with a "wrist-action" shaker
set on the maximum setting. Debris was removed by low-speed
centrifugation (50
x g for 15 min at 4°C). The bacteria
were pelleted by high-speed centrifugation (3,650
x g for 15
min at 4°C) and resuspended in 1 ml of phosphate-buffered
saline. A 0.5-ml aliquot was transferred to two 1.5-ml microfuge
tubes, and the bacteria were pelleted once again by high-speed
centrifugation (3,650
x g for 15 min at 4°C). The bacterial
pellet for one tube was resuspended in Superbroth (
48) with
15% glycerol and stored at -80°C, whereas nucleic acid was
extracted from the second bacterial pellet as described for
DNA extraction.
Viable counts of bacteria present in poultry litter.
Frozen bacterial suspensions were serially diluted in buffered saline gelatin (12). The dilutions were plated on MacConkey, mEnterococcus, and mannitol-salt agar for enumeration of gram-negative enterics, enterococci, and staphylococci, respectively. Dilutions were also plated on brain heart infusion (BHI) agar to obtain the total aerobic plate count for each litter sample. The litter samples, diluted 10-4 to 10-7, were plated on the MacConkey or Enterococcus agar; those diluted 10-8 to 10-11 were plated on BHI or mannitol-salt agar. Plates were incubated overnight at 37°C. Selective agar medium was acquired as dehydrated powder from Difco (Detroit, Mich.).
The environment of four, commercial broiler farms was sampled for Salmonella by using drag swabs: gauze pads soaked with double-strength skim milk that were then dragged across the birds' bedding material (9). Each swab was then placed in 100 ml of tetrathionate brilliant green broth (Difco) and incubated at 41.5°C for 18 h (6). A loop full of the tetrathionate brilliant green enrichment broth was streaked onto a XLT4/BGN biplate (Difco), followed by incubation at 37°C overnight (24).
DNA extraction.
The bacteria collected from chicken litter were resuspended in cell lysis solution (40% sucrose, 10 mM Tris-HCl, 1 mM EDTA, 100 mM NaCl, lysozyme [4 mg/ml], lysostaphin [30 µg/ml], RNase [250 µg/ml]) and incubated at 37°C for 1 h. After incubation, bacteria were pelleted by high-speed centrifugation (13,000 x g for 15 min at 4°C) and then resuspended in 10 mM Tris-1 mM EDTA. The subsequent steps were done according to the CTAB procedure for isolating bacteria DNA (2). DNA concentration (1.3 µg µl-1) was measured by using a Beckman DU640 spectrophotometer (Beckman Instruments, Inc., Fullerton, Calif.). Unless otherwise indicated, chemical reagents were purchased from Sigma Chemical Co. (St. Louis, Mo.).
PCR detection of pathogens and antibiotic resistance genes.
DNA extracted from litter was used in PCR to detect the presence of Salmonella spp. (34), Escherichia coli O157 (37), Campylobacter spp. (14, 21), Yersinia spp. (58), Listeria spp. (8), pathogenic E. coli (17, 46), and toxigenic staphylococci (29) and Clostridium perfringens (64). The specific genes, primer sequences, conditions, expected size of each amplicon, and PCR references are given in Table 1. Litter DNA was used at 2.5 and at 25 ng/10 µl of PCR volume. Clinical isolates from the veterinary diagnostic laboratory served as positive controls for the PCRs.
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TABLE 1. PCR detection of potential pathogens among DNA sequences extracted from the bacterial microflora of broiler chicken littera
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PCR was also used to detect the presence of class 1 integrons
and associated cassettes. The class 1 integrase (
int1I) was
detected by PCR as described by Goldstein et al. (
20) by using
litter DNA at 2.5and at 25 ng/10 µl of PCR volume. Associated
antibiotic resistance cassettes were amplified by using 5'-3'
conserved sequence (CS) PCR with primers targeted to the 5'
and 3' conserved regions of the class 1 integron sequences (
33).
The resistance cassettes were detected in a PCR-enzyme-linked
immunosorbent assay (ELISA) as previously described (
43). This
consisted of the 5'-3' CS PCR by using digoxigenin-labeled nucleotides
(Roche Molecular Biochemicals, Indianapolis, Ind.) and biotinylated
cassette-specific probes. The probe sequences and specificities
are shown in Table
2; all of the probes possessed a
Tm of 46
to 51°C and were used at high stringency (50°C annealing
temperature). The 5'-3' CS PCR mixture (50 mM Tris [pH 8.3],
3 mM MgCl
2, 0.25 mg of bovine serum albumin/ml, 0.5% Ficoll)
contained a 0.1 fM concentration of each 3'-biotinylated probe.
After 30 cycles the amplicons were denatured by incubation for
1 min at 96°C with probe annealing for 15 min at 50°C.
Probe-amplicon hybrids were captured in streptavidin-coated
wells and detected by using anti-digoxigen antibody-conjugate
as described by the manufacturer (Roche Molecular Biochemicals).
Oligonucleotide probes were pooled in the hybridization step
of the PCR-ELISA in order to detect resistance genes of certain
antibiotic classes.
Salmonella enterica serovar Typhimurium
DT104, which possesses two class 1 integrons containing
blaP1 or
aadA1, was used as the positive control for the aminoglycoside
and ß-lactam resistance pool (
7). Plasmid R388, which
possesses a class 1 integron containing
dfrB2, was used as the
positive control for the trimethoprim resistance pool (
56).
Tn
1696, which possesses a class 1 integron containing
cmlA,
was used as the positive control for the chloramphenicol resistance
pool (
4).
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TABLE 2. PCR primers and oligonucleotides used to probe for class 1 integrons and integron-associated antibiotic resistance cassettes among DNA sequences extracted from the bacterial microflora of broiler chicken littera
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PCR for construction of 16S rDNA clone libraries.
For construction of the 16S rRNA gene clone libraries, three
sets of primers, which target the domain
Bacteria, were used
(
26): primer set A, 8F [(5'-AGA GTT TGA TCC TGG CTC AG-3')/1492R]
[5'-TAC GG(C/T) TAC CTT GTT ACG ACT T-3']; primer set B, 8F/1522R
(AAG GAG GTG ATC CAN CCR CA); and primer set C, 8F/926R (ACC
GCT TGT GCG GGC CC). Primer 1492R was synthesized as a mixture
of oligonucleotides with either T or C at position 1497 (
E. coli numbering). Primer sets A and B are frequently used in
molecular diversity studies because they result in a nearly
full-length 16S rDNA product and are considered universal for
the domain
Bacteria and for the prokaryotes, respectively (
32).
Primer set C was used to minimize the effect of template concentration
on PCR bias (
11). The final reaction mixture included 25 or
100 ng of template DNA with primer set C and 25 ng with other
primer sets, 1' PCR buffer, 2.0 mM MgCl
2, a 0.2 mM concentration
of deoxynucleoside triphosphate, a 1 µM concentration
of each primer, and 0.05 U of
Taq DNA polymerase (Roche Molecular
Biochemicals) in a final reaction volume of 25 µl. Initial
DNA denaturation was performed at 94°C for 2 min in a PTC200
thermocycler (M. J. Research, Inc., Watertown, Mass.), followed
by 15 cycles of denaturation at 94°C for 1 min, annealing
at 54°C (primer set A), 48°C (primer set B), and 58°C
(primer set C), respectively, for 30 s, and then elongation
at 72°C for 1 min, which was followed in turn by a final
elongation at 72°C for 10 min. Fifteen cycles of PCR were
performed rather than 30 cycles to minimize the risk of certain
16S rDNA types being preferentially amplified (
63). Three separate
PCR amplifications were performed to minimize potential bias
due to PCR drift (
11) and to increase the DNA yield for subsequent
cloning. The nine PCRs generated with the three PCR primer sets
were pooled together for cloning. The amplified PCR products
were purified with the Wizard PCR product purification kit (Promega,
Madison, Wis.). The purified products were ligated into pGEM-T-Easy
(Promega). Ligation was done at 4°C overnight, followed
by transformation into competent
E. coli JM109 cells by heat
shock (45 s at 42°C). The clones were screened for

-complementation
of ß-galactosidase by using X-Gal (5-bromo-4-chloro-3-indolyl-ß-
D-galactopyranoside)
and IPTG (isopropyl-ß-
D-thiogalactopyranoside) (
2).
Plasmid extraction and sequencing.
DNA preparations for sequencing were made with the QIAprep spin plasmid kit (Qiagen, S.A.) as specified by the manufacturer. Plasmids were eluted with 50 µl of water, and the products were stored at -70°C. Sequencing reactions were performed with a PE-ABI BigDye terminator cycle sequencing kit (Applied Biosystems, Foster City, Calif.) as described by the manufacturer, and electrophoresis and readout were done with an ABI Prism 3700 DNA analyzer (Applied Biosystems). Primers T7 and SP6 were used in the sequencing reactions to sequence both strands of each PCR product.
Analysis of DNA sequences.
Resulting DNA sequences were edited to exclude primer-binding sites and ambiguous bases and assembled into contiguous sequences (570 to 650 bp) by using the Sequencher program, version 4.10 (Gene Code Corp., Ann Arbor, Mich.). The resulting DNA sequence information was analyzed by using the programs FASTA (47) and BLAST (1). Chimeric sequences were detected as described by Suau et al. (55). The estimate of sample size and coverage were conducted according to the formula for coverage as described by Good (22) and applied in quantitative comparisons of 16S rRNA gene sequence libraries as described by Singleton et al. (52). The same definition for the variables in the formula Cx = 1 - (Nx/n) used by Singleton et al. (52) was used here, i.e., where Cx is the "homologous" coverage of sample X; Nx is the number of unique sequences, and n is the total number of sequences in the sample. Coverage was considered at two levels of minimum match percentage, 97 and 99%, respectively, for 16 S rDNA sequences (350 to 410 bp), which resulted from the three primer sets (38, 55). Based on 231 consistent sequences, Nx was analyzed by using Sequencher program, and Cx was calculated by the method of Good (22). Representative sequences were submitted to GenBank as accession numbers AY080963 to AY080994.

RESULTS AND DISCUSSION
Poultry litter used in the present study contained 10
9 aerobic
bacteria per g of material as determined by CFU on BHI agar.
Staphylococci accounted for the majority of the total aerobic
plate counts (13%), whereas enteric bacteria such as enterococci
(0.1%) and gram-negative enterics (0.11%) were only minor components
of the total bacteria cultured from poultry litter. The levels
of total bacteria in our samples were much higher than those
detected by Martin and McCann (
36) in which the total bacterial
counts ranged between 1,200 and 8.4
x 10
7. Martin and McCann
also detected low plate counts of gram-negative bacteria and
high plate counts of
Staphylococcus spp. (
36). The plate counts
for
Enterococcus spp. and facultative anaerobic, gram-negative
bacteria appeared to reflect their relative abundance among
the chicken's normal flora of the gastrointestinal tract (
66).
However, accurately discerning the microbial composition is
limited by the chosen culture methods and the physiological
state and nutritional or atmospheric requirements of the microorganism(s)
targeted for growth. In order to obtain an alternative assessment
of the major microorganisms that inhabit chicken manure, we
generated 16S rDNA libraries from the microbial community DNA
of broiler litter.
A total of 340 contiguous sequences originating from the 16S gene were retrieved from the 16S clone library. The library consisted of 265 sequences based on primer set A (78% of total clones) and 31 sequences from primer set B (9%) and 44 sequences from primer set C (13%). The sequences from the three primer sets were very similar in composition and abundance (data not shown); therefore, we used the contiguous sequences from primer set A as the main source for sequence analysis. For calculation of the coverage, 231 consistent sequences (350 to 410 bp) were analyzed. There were 84 and 103 distinct sequences at similarity levels of 97 and 99%, respectively. The coverage calculated for the 231 sequences was 71% ± 2% at 97% identity and 62% ± 3% at 99% identity, suggesting that the majority of unique sequences of the microbial community in chicken litter were reflected.
Based on analysis of the 340 clones isolated from the 16S rDNA libraries of bacteria collected from broiler litter, we identified four major phyla. These phyla included low and high G+C gram-positive bacteria, proteobacteria, and the CFB (Cytophaga-Flexibacter-Bacteroides) group (Table 3). A total of 12 families or groups and 31 genera were identified among the 16S rDNA sequences analyzed. The broiler litter bacterial microbiota consisted predominantly of gram-positive bacteria (87%), with low G+C gram-positive bacteria accounting for 62% of the total 16S rDNA sequences in the libraries. The low G+C gram-positive bacteria consisted of four families or groups represented by 15 genera. Identification of members of Lactobacillaceae and Clostridiaceae was not unanticipated for the environment expected to contain these fecal bacteria (66). However, we did not anticipate finding that 2% of the 16S rDNA sequences from the library would be Enterococcus specific, since this group comprised only a small percentage of the total cultivatable aerobic bacteria in poultry litter. There was good agreement between 16S rDNA frequencies and culture results for staphylococci. However, unlike Martin and McCann (36), we did not detect S. xylosus as the prevalent staphylococcal species (Table 3). Several 16S rDNA sequences were identified with homology to other gram-positive bacteria, including Arthrobacter, Brevibacterium, and Cellulomonas spp. (Table 3), organisms that may be involved in the decomposition of organic material including wood (18, 50, 57). 16S rDNA sequences were also identified with homology to sequences for Corynebacterium urealyticum, Desulfotomaculum, and Globicatella sulfidofaciens, which are anaerobic, reductive bacteria (31, 51, 59).
We identified many 16S sequences demonstrating homology to bacteria
potentially pathogenic for humans and animals (Table
4). Several
sequences with homology to
Bordetella spp. were detected, however
we could not identify the species since there are no discernible
differences in 16S rDNA among
Bordetella pertussis,
B. parapertussis,
B. bronchisepticum, and
B. avium (
39). None of the poultry farms
from which the litter was collected had reported any present
or past history of avian bordetellosis (
B. avium), and there
are no published reports of the human pathogenic bordetellae
(
B. pertussis and
B. parapertussis) associated with birds. These
sequences may represent strains causing subclinical avian disease
or uncharacterized species. Other sequences were homologous
to the organisms potentially virulent for animals, including
Brevibacterium avium (
45) and
Clostridium sp. (
35,
62).
Clostridium perfringens is an important cause of necrotic enteritis in broilers,
and it is generally managed or controlled with growth-promoting
antibiotics (
19,
35). Clostridia can also cause gangrenous dermatitis
in poultry (
62). However, sequences related to
Clostridium sp.
exhibited less than 96% homology to known
Clostridium sp., suggesting
that they may be uncharacterized strains. Staphylococcal 16S
rDNA sequences were also frequently identified from the poultry
litter library. The
Staphylococcus 16S sequences were homologous
to
S. cohnii,
S. succinus,
S. lentus, and
S. arlettae, and we
did detect 16S rDNA sequences exhibiting high homology to
S. aureus, the etiological agent of arthritis and gangrenous dermatitis
in chickens (
10,
41) and foodborne gastroenteritis in humans.
However, PCR for specific enterotoxin or toxic shock syndrome
toxin genes did not indicate that pathogenic staphylococci were
prevalent in the litter. Although not pathogenic themselves,
there are concerns that normal flora staphylococci and enterococci
of animals may serve as a reservoir for transmitting antibiotic
resistances to human commensal flora (
30).
The molecular characterization of microbial ecosystems often
involves the random PCR amplification, cloning, and sequencing
of the 16S rDNA library. The chance that a unique clone was
detected depends on its abundance in the library and on the
total number of clones sequenced. Therefore, the limitation
of this technique is that this method may miss unique clones
representing very minor populations. We therefore chose PCR
to target specific pathogens, which may exist as minor components
of the litter microbiota (Table
1). PCR screens did not detect
zoonotic or bovine specific pathogens including
Salmonella,
Campylobacter,
Yersinia, and
Listeria spp. or virulence genes
associated with pathogenic
E. coli or
S. aureus. Salmonella was neither detected by PCR nor culture in the present study,
and our results agree with those of a previous study (
36). These
findings were not entirely unexpected since the prevalence of
Salmonella spp. on poultry farms is sporadic (
34). Litter DNA
did not interfere or reduce the sensitivity of PCR since detection
limits were similar for litter DNA spiked with purified
Salmonella DNA versus purified
Salmonella DNA alone (data not shown). The
absence of enterotoxigenic staphylococci, enterotoxigenic
E. coli, enteropathogenic
E. coli, and
E. coli O157 in poultry
litter was not unexpected due to their reported low frequency
or absence in poultry flocks (
5,
25,
28). We also did not detect
genes associated with
E. coli K99, F41, or EaeA adhesins that
are required for
E. coli's colonization of a bovine or porcine
host (
13). However, it should be noted that, depending on the
primers, the detection limits of PCR range from 10
4 cells to
as few as 10 cells. Therefore, these organisms may be present
in litter but well below the detection limit of PCR.
From our analysis of poultry litter microbiota, many bacterial species were identified that may be actively involved in composting of organic matter, and this may explain absence of several important veterinary and human pathogens from this environment. Poultry farms may or may not remove the old litter before placement of every new flock. Several poultry farms visited in the present study remove poultry litter only twice a year, placing fresh litter shavings on top of the old litter, which may allow the microbial activity to sufficiently compost the litter (27, 28). Comparisons of litter microflora from farms with different litter management practices may identify practices or conditions that reduce or eliminate harmful vegetative bacteria.
However, we did detect antibiotic resistance cassettes associated with class 1 integrons. The integron is potentially a major agent in the dissemination of multidrug resistance among gram-negative bacteria. The integron contains an integrase gene and a site-specific integration site where the integrase can link antibiotic resistance gene cassettes in tandem in the integration site if the circular cassette molecules possess a 59-base element (33, 49). More than 60 distinct antibiotic resistance gene cassettes have been characterized within the class 1 integron, and as many as 7 of them have been found in a single integron at one time (15, 40). In our experiments, PCR targeting of the class 1 integrase gene produced a product of the appropriate size, indicating that integrons were present in the community DNA. Past studies have shown a high prevalence of integrons in E. coli isolated from broiler chickens (3, 20), and culture of the litter in our study showed that coliforms were present at 250,000 CFU/g of litter. Bass et al. (3) demonstrated that the streptomycin resistance gene cassette, aadA1, was commonly found in the class 1 integrons possessed by avian E. coli. The class 1 integron cassettes can be amplified by PCR with primers anchored in the 5' and 3' conserved regions of the class 1 integron (33). We designed oligonucleotide probes to detect the presence of cassettes associated with aminoglycoside, ß-lactam, chloramphenicol, and trimethoprim resistance (Table 2). These were used to determine whether the litter DNA contained integron associated-antibiotic resistance genes, which are potentially transferable to other ecological environments. We did not detect cassettes encoding ß-lactam resistance or trimethoprim resistance but did detect the presence of genes encoding aminoglycoside resistance and chloramphenicol resistance. Antibiotics of these classes were not administered to the flocks raised on the litter used in the present study. In fact, chloramphenicol has not been used in food animals for more than 15 years, and no formulation exists on the market for its administration to flocks of birds. The detection of these resistance cassettes further illustrates the potential for antibiotic resistance genes to be present at detectable levels in environments in which the antibiotic's administration may not be recent. Subsequent application of poultry litter, as either a soil amendment or cattle feed, may not immediately negatively impact cattle or humans. However, its use may disseminate increasing loads of resistance genes that may be amplified in new environments.

ACKNOWLEDGMENTS
This work was funded by the U.S. Department of Agriculture (USDA
NRICGP 99-35212-8680 to J.J.M. and USDA NRICGP 01-35212-10877
to C.H.) and by USDA Formula Funds (M.D.L. and B.G.H.).

FOOTNOTES
* Corresponding author. Mailing address: Medical Microbiology and Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602. Phone: (706) 542-5778. Fax: (706) 542-0059. E-mail:
leem{at}vet.uga.edu.


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Applied and Environmental Microbiology, February 2003, p. 901-908, Vol. 69, No. 2
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.2.901-908.2003
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